At one point, Lego didn't really support putting a stud into a Technic hole, stating that it caused too much stress on the bricks. And for a while, there were no sets where that's the case. However, ...

LEGO purism is the idea that LEGO constructions should be made with only with official Lego blocks with no modifications. Other bits of plastic, papercraft, metal supports, and so on are frowned upon.
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Which Lego set is considered to be the most valuable set to own?
I'm not talking about the most expensive set, and clearly it has to be out of production already, but when trying to collect complete ...